You are billed for your hosting package on the 1st of each month, this is useful for people who get paid at the end of each month and don't like bills coming out in the middle of a month.
When you first sign up for a hosting package you will pay for your first month upfront plus the rest of the current month, this will enable us to make sure your bill is set to start recurring on the 1st of each month from then on.
This pro-rata is a one-off.
Your first bill includes 1st month upfront plus the rest of the current month.
Your next bill will be for the month after that, which is usually 2 months away it's displayed in brackets like (Pro Rata 01/10/2015)